The invention relates to recording and reading data from a magnetic recording tape. More particularly, the invention relates to tape servo systems, to arrangements of servo and data tracks on a magnetic recording tape, and to methods and apparatus for sensing tension in a magnetic recording tape.
Accurate tension control while moving tape in a reel-to-reel tape transport is needed to maintain proper contact between the tape and the read-write head, low tape wander and uniform tape packing on the takeup reel.
Typical prior art tension sensors are implemented with some sort of forced lever arm which protrudes into the tape path. For example, U.S. Pat. No. 5,395,066 to Yokoyama et al. (incorporated herein by reference) discloses a tape tension sensor that uses a movable pin, and a tape tension control system that adjusts tape tension responsive to the tape tension sensor. With prior art tension sensors that used a forced lever arm, the deflection of the lever arm is then an indication of the tape tension and the tension variation. The force of the element protruding into the tape path is balanced by the normal force created by the tape tension. As the tape tension changes, the forces are unbalanced and the resulting force changes the amount of protrusion into the tape path by the element until a new equilibrium is established.
The deflection of the protruding element can be measured with a variety of position measuring methods such as capacitance detection, laser interferometry and piezo-electric sensing.
A difficulty with this prior art approach is that the mass of the protruding element must be moved in response to the unbalanced force created by a variation in the tape tension. The movement of this mass limits how rapidly the tension force can be countered by the protrusion force. The result is a bandwidth limitation where beyond a certain frequency of tension variation, the sensor will fail to indicate the true tension variation.
Because some sort of spring is required to create the protrusion force which is balanced by the tension force, a spring-mass system is created. Such spring-mass systems will have a resonant characteristic which can limit the useful bandwidth to some fraction of the frequency of the resonance.
The sensing of the position of the protruding element can also present difficulties. The sensing is typically analog in nature and will have noise, non-linearities, environmental drift and calibration errors.
Attention is directed to various issued U.S. patents. U.S. Pat. No. 5,392,976 to Nelson et al. (incorporated herein by reference) discloses a tape duplication system in which a pressurized fluid is used for sensing tape tension based on tape width, among other things.
U.S. Pat. No. 5,982,711 to Knowles et al. (incorporated herein by reference) is assigned to the assignee of the present invention and discloses calculating lateral expansion of tape by comparing servo signals from two servo bands.
U.S. Pat. No. 5,262,908 to Iwamatsu et al. (incorporated herein by reference) relates to a tracking control device that has at least two servo signal reproducing heads for reproducing servo signals for tracking from a plurality of servo tracks. The Iwamatsu et al. patent does not relate to tension sensing.
U.S. Pat. No. 5,432,652 to Comeaux et al. (incorporated herein by reference) discloses the use of multiple servo tracks on a magnetic recording tape.
The invention provides a tension sensor that uses servo signals read from the read head, to avoid the problems of prior art tension sensors.
Another aspect of the invention provides a method of determining tape tension comprising sensing a change in lateral width of the tape and determining change in the tension of the tape in response to the sensed change in lateral width.
Another aspect of the invention provides a tape drive for use with a magnetic recording tape, the tape drive including a system for determining the amount of tension on the magnetic recording tape, the magnetic recording tape having a length in the direction of tape travel and a width in the direction perpendicular to the length, the tape having first and second servo bands thereon spaced apart in a direction transverse to the length of the tape, the tape drive comprising servo elements configured to read the first and second servo bands on the tape; and a servo processor configured to determine changes in distance between the first and second servo bands, and to detect changes in the amount of tension along the length of the tape based on the determined changes in distance between the first and second servo bands.
Another aspect of the invention provides a method of determining tension on a magnetic recording tape that has a length in the direction of tape travel and a width in the direction perpendicular to the length, the tape having first and second servo bands thereon, the method comprising reading the first and second servo bands on the tape; determining the distance between the first and second servo bands; and determining the amount of tension along the length of the tape in response to the determined distance between the first and second servo bands.